Isotropic earth models have been in standard use in the oilfield industry for many years; not because isotropy is a good assumption, but because anisotropic measurements have not been available. There are three basic types of formations: formations with identical elastic properties in all three spatial directions are called isotropic, formations with identical elastic properties in a plane (often the bedding plane) but a different property in the direction normal to the plane of symmetry are called transversely isotropic (TI), and formations with different properties in all three dimensions are called orthotropic (also known as orthorhombic). Two independent moduli (or stiffness coefficients in the stiffness matrix characterizing the formation) are used to characterize isotropic formations, five independent moduli are used to characterize TI formations, and nine independent moduli are used to characterize orthotropic formations.
Much of the motivation to determine the TI moduli focuses on determining a better stress profile to be used in geomechanics for drilling and completion solutions in gas shales, which have been made productive in recent years based on methods such as hydraulic fracturing. Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic propagation because of their low permeability and anisotropic micro-structure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology, and pore pressure from seismic data, and to understand time-depth conversion errors and non-hyperbolic move-out.